Piston driven automotive air conditioning compressors all draw in and compress a mixture of refrigerant vapor and entrained lubricant within a close fitting cylinder, as the piston is driven axially back and forth. The lubricant entrained in the refrigerant vapor provides a lubricating film to those moving parts and interfaces to which it is exposed. In extreme conditions, some liquid refrigerant may be drawn into the cylinder, which is not nearly so compressible as vapor. This settles preferentially in the lower cylinders, and its resistance to compression as the piston is driven forward is generally referred to as "slugging."
Piston compressors typically fall into one of two broad categories based on the piston drive means, swash plate or wobble plate. Any piston compressor has to have a sliding interface between the piston and the drive means, since the drive means rotates with the shaft and the piston does not, and that sliding interface is located differently within these two broad compressor types. In a swash plate compressor, a single slanted plate rotates one to one with the drive shaft, and the edge of the one-piece plate slides through a slot in the back of each piston, supported by sliding shoes. An example may be seen in co assigned U.S. Pat. No. 5,720,215.
In a wobble plate compressor, an example of which is illustrated in FIG. 1, a compressor housing 10 encloses a crankcase chamber 12 located behind a cylinder block 14. Cylinders 16 formed in the block 14 are arrayed around the axis of the central rotating drive shaft 18. The drive means consists of two plates, a primary plate 20 that rotates directly, one to one, with the shaft 18, and a secondary plate 22 supported by rolling bearings 24 on the primary plate 20. The primary plate 20 drives the secondary plate 22 back and forth in a nutating or "wobbling" motion, but the secondary plate 22 does not rotate itself. Therefore, each piston 26 can be directly connected to the secondary plate 22, typically by a rod 28 with a ball 30, 32 at each end. Each ball 30, 32 is received in a socket joint, one socket 34 formed in the secondary plate 22, and one socket 36 formed integrally with the back of the piston 26. Piston 26 is typically formed of an aluminum alloy sufficiently malleable to allow the socket 36 to be integrally deformed around ball 30. As the plates 20 and 22 nutate, the connecting rod 28 tilts on and off the axis of the cylinder 16 as the balls 30, 32 twist within their respective sockets 34, 36.
Each type of compressor faces unique problems and issues. In the swash plate compressor, the piston is much larger, with a cylindrical front surface or head that does the actual compressing within the cylinder, and a rear body that extends from the head all the way back to the drive plate. This represents a good deal of material and mass, and several patents provide hollow or near hollow piston designs to remove weight. The rear of the piston body extends out of the cylinder and into the compressor housing or crankcase chamber, where it can turn far enough to rub on the housing wall. The above noted patent provides a piston designed to minimize that rubbing wear. Lubrication of the sliding interface between shoes and swash plate is an issue, but the interface is generally well enough exposed to refrigerant vapor and lubricant within the crank chamber to avoid excessive wear. If not, either the shoe or the plate surface can be coated with any number of existing wear resistant bronze alloys, by several conventional methods. The rotating joint between the shoes and the pistons is also generally well exposed to a refrigerant-lubricant mist, since only half or less of the spherical surface area of the shoe is embedded into the piston socket.
In a wobble plate compressor, the piston is much shorter axially, only about the size of the front head of a swash plate compressor piston. It is inherently lighter and simpler to manufacture, and does not extend out of the cylinder at all. The most significant problem recognized in the prior art relevant to a wobble plate compressor piston is the problem of friction and wear in the piston-connecting rod ball and socket joint. Since the socket has to wrap significantly around and past the equator or center plane of the connecting rod ball, the turning interface between ball and socket is not well exposed to the refrigerant-lubricant mist in the crank chamber. In the event that slugging increases the pressure on the piston, arid the consequent normal contact force between the ball and the socket, the increased frictional force in the joint can stress the connecting rod as it tilts off the cylinder axis.
The prior art recognizes the problem of providing lubrication to the piston ball and socket joint. Since the piston is exposed at the front to the compression space in the cylinder, that represents a possible source of lubricant for its ball and socket joint. However, the industry is apparently unanimous in its judgment that the only practicable means of introducing lubricant from the cylinder compression space to the piston's socket joint is by providing an indirect passage from the cylinder compression space to the socket, so as to throttle down the pressure. For example, in the design disclosed in Japanese UM No. 01-71178, shown in FIG. 2, the piston 38 has a series of oblique passages 40 cut through the side wall, just under the piston ring seals 42, and opening into the socket 44. Oddly, the socket 44 as disclosed does not wrap around the ball 46 sufficiently to even be workable, although other figures show it differently. The clear purpose is to open a path from just under the seals 42 to the socket 44 for lubrication, but without being exposed directly to the high pressure in front of the piston 38.
This same intent is evidenced in the design shown in U.S. Pat. No. 5,137,431, shown in FIG. 3. Here, the design in the Japanese UM noted above is recognized, but it is claimed that the path shown there is still too direct. It is claimed that "Smooth movement of the ball portion within the spherical concavity is prevented by the undesirable high pressure of the refrigerant gas. Consequently, abnormal wearing of the inner surface of the spherical concavity and the outer surface of the ball portion is experienced." It is also claimed that such high pressure would actually decrease the amount of oil reaching the socket. Accordingly, it is proposed to provide a similar oil passage 48, but opening between the two ring seals 50, so as to throttle down the high pressure from the cylinder before it reaches the socket joint. In another embodiment, the diameter of the passage is actually decreased to a very small size before entering the socket, so as to further throttle down the pressure.
As to the opposite socket joint, that formed in the secondary drive plate, many designs do show a central hole opening to the center of the ball socket. An example may be seen in U.S. Pat. No. 4,747,203. The hole is not intended as an oil or lubricant passage, however, despite its appearance, but is simply a remnant of the method by which the socket is formed. A push pin impacts the ball when the integral socket is formed, in order to create a small gap between the ball and socket, although the shape and detail of the claimed gap is not disclosed. The through hole is simply left when the pin is withdrawn. There would be no pressure differential within the crankcase to force oil into such a through hole, in any event, so that its effect in improving lubrication would be minimal.